1. Field of the Invention
The present disclosure relates generally to the fields of biology, medicine and pathology. More particularly, it concerns the identification and production of cyclized peptides that can protect neurons fron Aβ toxicity.
2. Description of Related Art
Alzheimer's disease (AD) is the most common form of dementia, affecting more then 35 million people worldwide. Intracellular neurofibrillary tangles and extracellular senile plaques, primarily in the hippocampus and cerebral cortex, are two characteristic features of the disease. The main component of the extracellular plaques is β-amyloid (Aβ), a proteolytic product of amyloid precursor protein (APP). When cleaved from APP, Aβ spontaneously self-assembles into soluble oligomers that progress into insoluble fibrillar aggregates. Although the precise mechanism of AD pathogenesis is unknown, several studies have suggested that the aggregation of Aβ plays a key causal role, and that the soluble oligomers are the most toxic Aβ species (Roychaudhuri et al., 2009, Haass and Selkoe, 2007; Lesnê et al., 2006).
Numerous small molecules that can alter Aβ production and/or aggregation have been explored as therapeutic agents against AD, although to date none have been clinically effective (Karran et al., 2011). As an alternative approach, the use of peptides and peptidomimetics that bind to Aβ and alter Aβ aggregation was introduced, since peptides have potential advantages over small molecules in terms of better target affinity and specificity (Craik et al., 2013). In general, peptides that bind to Aβ have been either designed based on self-complementary domains, or found by screening random-peptide libraries (Takahashi and Mihara, 2008, Tjernberg et al., 1996; Taylor et al., 2010; Soto et al., 1998). Alternatively, one could design a peptide by mimicking the binding epitope of a complementary binding protein. Molecular recognition involving proteins is often mediated by relatively large interfaces with secondary structural motifs such as β-hairpins and α-helices, and thus has the potential for higher specificity than small-molecule binders. Peptides mimicking the three-dimensional structure of protein binding sites have been recognized as a promising alternative, as inhibitors of protein-protein interactions, to large biologics such as antibodies (Robinson, 2008; Eichler, 2008). This approach is in particular useful when one has precise structural information about the protein-protein interaction site.
Transthyretin (TTR) is a stably-folded homotetrameric transport protein that circulates in blood and cerebrospinal fluid. TTR has been shown to be neuroprotective in AD mouse models (Stein and Johnson, 2002; Buxbaum et al., 2008; Stein et al., 2004). The inventors and others have shown that TTR binds to Aβ and inhibit its toxicity in vitro (Giunta et al., 2005; Li et al., 2011). Inhibition of toxicity is mediated at as low as 1:100 TTR:Aβ molar ratio, implying that TTR is selective for toxic Aβ oligomers (Yang et al., 2013). A peptide mimicking the epitope of Aβ-TTR interaction might therefore have a great therapeutic potential.
Previously, the inventors identified two putative binding sites on TTR: strand G in the inner hydrophobic pocket and solvent exposed EF helix (Du et al., 2012). They further analyzed the binding domain on strand G to determine the minimum requirement for binding, and synthesized a 16-mer linear peptide, G16, with the sequence of this binding domain. G16 bound Aβ and was protective against Aβ toxicity in vitro, but required a much higher concentration than TTR to afford the same level of protection. In addition, G16 differed substantially from TTR in its effect on Aβ aggregation (Cho et al., 2014).